Low-temperature electron paramagnetic resonance ...

American Mineralogist, Volume 96, pages 654–658, 2011

Low-temperature electron paramagnetic resonance studies on natural calumetite from Khetri copper mine, Rajasthan, India G.N. Hemantha Kumar,1 G. Parthasarathy,2 and J. Lakshmana Rao3,* 2

1 Government Polytechnic for Women, Nellore-524 003, India National Geophysical Research Institute, Council of Scientific and Industrial Research, Hyderabad-500 007, India 3 Department of Physics, Sri Venkateswara University, Tirupati-517 502, India

Abstract We report here for the first time low-temperature (123–295 K) electron paramagnetic resonance (EPR) spectroscopic data on naturally occurring calumetite [Cu(OH,Cl2)∙2H2O] from the Khetri copper complex, Jhunjhunu district, Rajasthan, India. The sample was characterized by X-ray powder diffraction (XRD), differential thermal analysis (DTA), and thermogravimetric (TG) analysis techniques. DTA scans show two endothermic reactions at about 378 and 598 K, which are attributed to dehydration of the sample. TG data indicate weight loss of up to 24 wt% in the temperature region between 600 and 700 K. The room-temperature EPR spectrum exhibits parallel and perpendicular components centered at g|| = 2.26 and g⊥ = 2.10, respectively. The evaluated spin-Hamiltonian parameters indicate that these resonance signals are characteristic of Cu2+ ions in distorted octahedral symmetry. The population difference (N) between the Zeeman levels has been evaluated and is found to increase with decreasing temperature. Keywords: Calumetite, EPR spectra, DTA, hydroxy mineral

Introduction Mineralogical and spectroscopic studies of the copper chlorides and related minerals have attracted attention because of the formation of these compounds in bronze corrosion as well as in painting pigments. Calumetite was first discovered in Calumet, Michigan, but has since been observed in other mines near the Calumet mine (Williams 1963). It was suggested that calumetite is not a post-mine evaporite because of the mode of occurrence. Calumetite is insoluble in ammonia and water, and soluble in cold dilute acids. Other copper minerals have been linked to calumetite and include copper, cuprite, malachite, atacamite, paratacamite, and buttgenbachite. Synthetic calumetite is prepared with ammonium chloride by means of the lime blue recipe. Calumetite as a pigment has been detected in the products of historical recipes for lime blue and occurs in corrosion products of bronze (Scott 2000) and hence has proven to be useful in preserving archaeological paintings on canvas and fresco (Christopher and Kurt 2003). Copper chloride is also used as a catalyst for synthesizing diethyl carbonate (Punnoose et al. 2002; Xiong et al. 2009). To the best of our knowledge, there are no previous reports available in the literature on the thermal or electron spin spectroscopic properties of this mineral. The distribution of paramagnetic ions on different sites in minerals can be easily determined by electron paramagnetic resonance (EPR), which has proven to be a sensitive and discriminating spectroscopic technique (Pan et al. 2008; Kumar et al. 2009, 2010). In continuing our efforts in using this technique for understanding the short range structural parameters in hy* E-mail: [email protected] 0003-004X/11/0004–654$05.00/DOI: 10.2138/am.2011.3655

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drous minerals containing transition metal impurities (Kumar et al. 2009, 2010), we undertook a study of calumetite, a rare copper hydroxyl mineral, for the present investigation. EPR studies of Cu2+ ions in different mineral environments have been informative from both structural and catalytic aspects (Chao and Lunsford 1972). Samples with high-Cu content occur either as CuCl2 or as Cu2(OH)3Cl [paratacamite] (Leofanti et al. 2000). The former is a readily soluble Cu compound, not detectable by XRD owing to high dispersion of CuCl2. On the other hand, paratacamite is predominant in XRD patterns of samples with high-Cu concentrations. Leofanti et al. (2000) pointed out that traces of paratacamite in samples are products of slow hydrolysis of CuCl2, catalyzed by basic sites of the alumina surface. Blanco et al. (1973) compared EPR, solubility tests, and XRD results on a set of samples with Cu contents in the 0.8–10.5 wt% Cu range. They concluded that the species dominating in high-Cu content specimens is paratacamite. Low-temperature EPR spectra of CuCl2/activated carbon for different concentrations of Cu reveal two Cu2+ species: Cu2+-carbon and nanoclusters of CuCl2 precipitated during impregnation (Punnoose et al. 2002). These studies on copper halides motivated us to examine the electron paramagnetic resonance in calumetite and also to understand its thermal behavior to better understand the thermal phase stability of this rare copper hydroxyl chloride mineral. In this study, the temperature dependence of electron paramagnetic resonance on the naturally occurring calumetite from a copper mine, Rajasthan, India, is reported with the motivation of understanding the resonance signals arising from the transition metal impurities present in calumetite, at room temperature and at low temperatures, and also to interconnect the site symmetry and oxidation states.

Hemantha Kumar et al.: Low-temperature EPR studies on natural calumetite

Experimental methods Greenish, opaque, and hygroscopic samples of natural calumetite occur in the 2.2 Ga old basement rock. Palaeoproterozoic rifting of the basement produced linear troughs where copper mineralization took place. The geology and tectonic settings of this region are discussed by Sinha-Roy (2000). The samples were powdered using a porcelain mortar and pestle. The chemical composition of the sample was determined by an electron probe microanalyzer (EPMA). Energydispersive X-ray (EDAX) measurements were carried out by using scanning electron microscope (JEOL JSM-840 A) in EDAX mode with a filament current of 100 µA and an accelerating voltage of 20 kV. Five independent measurements were carried out and the average composition of the calumetite sample is presented here. The compositional analyses yielded the chemical formula of the studied sample as Cu(OH,Cl2)⋅2H2O. The powder X-ray diffraction (XRD) measurements were carried out on the powdered calumetite samples at room temperature using a Siemens D-5000 powder X-ray diffractometer with a HOPG graphite monochromator, and CuKα radiation with a wavelength of 0.15406 nm. To check the reproducibility, triplicate runs were made. Differential thermal analysis (DTA) and thermogravimetric (TG) studies were performed on powder samples using a Mettler Toledo Star System apparatus (Parthasarathy 2006). The temperature was measured with platinum sensors. Temperature precision and accuracy are ±0.1 K. Thermogravimetric method is used to quantify the percentage of hydroxyl/water content in the sample. The calibration and reproducibility of this apparatus is discussed elsewhere (Parthasarathy et al. 2001, 2002). A JEOL-FE-1X ESR spectrometer operating at the X-band frequency of 9.205 GHz with a field modulation of 100 kHz was used for the acquisition of EPR spectra, which were obtained as the first derivative of microwave absorption. The magnetic field was scanned from 0 to 5000 G and the field was swept at the rate of 1250 G/min. The modulation field width was set at 0.5 G and the microwave power was set at 5 mW. A powdered mineral sample of 100 mg was placed in a quartz tube for EPR measurements. EPR spectra were recorded on a chart paper using D-YT type recorder. CuSO4⋅5H2O was used as a reference material for the EPR studies. The EPR spectra of CuSO4⋅5H2O powdered samples were recorded at different temperatures as a reference to calculate the population difference between the Zeeman levels. EPR spectra of calumetite were also recorded at different temperatures in the 123–295 K range. The temperature was varied using a JES-UCT-2AX variable temperature controller. For low-temperature measurements, nitrogen gas was evaporated from liquid nitrogen in a metal Dewar; the evaporation rate was controlled automatically. The desired temperature can easily be obtained by applying cold air to the vicinity of the specimen. A temperature stability of ±1 K was obtained by waiting for about 30 min at the set temperature before recording the spectrum at each temperature.

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not attempt to index the peaks. It is worth mentioning here that atacamite Cu2(OH)3Cl also exists in the copper mine waste, but has a distinctly different XRD pattern with strongest lines at 5.48, 5.03, 2.27, and 1.606 Å. The present sample did not show any XRD lines corresponding to atacamite. Differential thermal analysis and thermogravimetric studies Figure 1 shows the differential thermal analysis (DTA) and thermogravimetric (TG) traces of calumetite at ambient pressure. The DTA trace shows two endothermic reactions at about 378 and 598 K, which are attributed to dehydration of the sample, which is typically observed in hydrous minerals (Parthasarathy 2006; Parthasarathy et al. 2001, 2003, 2007). The exothermic peak at about 650 K may be due to solid–solid phase transition or decomposition of the sample. TG analyses demonstrate weight loss of the sample up to 24 wt% in the temperature region of 600 to 700 K; the value agrees with the compositional data. Temperature induced dehydration occurs in two different stages, first at about 380 K, with a 5% weight loss and the second reaction at about 600 K with a weight loss of 10%. Furthermore, in situ XRD measurements at high temperature would be useful in understanding the nature of the phase transition. EPR spectral studies Figure 2 shows the EPR spectrum of natural calumetite recorded at room temperature. The spectrum exhibits resonance signals characteristic of Cu2+ ions. The Cu2+ ion with S = 1/2 has a nuclear spin I = 3/2 for both 63Cu (natural abundance 69%) and 65Cu (natural abundance 31%). The interaction of unpaired electron spin with the copper nuclear spin leads to a fourfold splitting of each electron spin state, which results in a four-line pattern in the EPR spectrum, i.e., four parallel and four perpendicular components would be expected. In the recorded spectrum, we observed an intense unresolved parallel component in the

Results and discussion Chemical analysis The composition of the prepared sample is determined as CuO (52.46 to 52.84 wt%); Cl (23.16 to 23.54 wt%) and the loss of ignition is about 24 wt%. The average of five different measurements yielded the chemical formula of the studied sample as Cu(OH,Cl2)⋅2H2O.

Powder X-ray diffraction Powder X-ray diffraction (CuKα radiation) patterns of the studied sample showed nine broad diffraction peaks at 7.50 Å (100), 3.76 Å (45), 3.42 Å (25), 3.30 Å (20), 3.00 Å (50), 2.46 Å (80), 2.30 Å (30), 2.00 Å (25), and 1.71 Å (30) (relative intensities in parentheses). The standard deviations for the peaks range from 0.005 Å for the peak at 7.5 Å to 0.002 Å for the peak at 1.71 Å. The observed diffraction peak positions are found to be in good agreement with the XRD pattern of calumetite (JCPDS Card number 15-669). The broadness of the Bragg peaks could be due to the nano-crystalline nature of the sample. As we do not know the crystal structure details of the calumetite, we did

Figure 1. Differential thermal analysis and thermogravimetric traces of calumetite at ambient pressure.

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Figure 2. EPR spectrum of polycrystalline calumetite at room temperature.

lower field region and well-resolved hyperfine perpendicular components in the higher field region. For Cu2+ ions, a regular octahedral site may not exist because the cubic symmetry is broken by an electronic hole in the degenerate dx2–y2 orbital that produces the tetragonal distortion (Jahn-Teller distortion). The EPR spectra of Cu2+ ions in the present work can be analyzed by using an axial spin-Hamiltonian of the form H = β[g||BzSz+g⊥(BxSx+BySy)]+A||SzIz+A⊥(SxIx+SyIy)

g|| > g⊥ > ge (free electron g value), we consider that the ground state for paramagnetic electron is the dx2–y2 orbital (2B1g state); the Cu2+ ion being located in distorted octahedral sites (D4h) elongated along the z-axis. From the observed EPR spectra, the perpendicular component of the hyperfine splitting constant is found to be A⊥ = 45G. The parallel component of the resonance signal is not resolved and hence A|| could not be determined. Figure 2 shows the asymmetric line, characteristic of Cu2+ ions in distorted octahedral sites. The reason for the distortion of the coordination sphere from octahedral is the stabilization of the 3d9 electron configuration through the Jahn-Teller effect. The observed spectra exhibit higher intensities, which can be attributed to greater amounts of Cu2+ ions present in the sample. Figure 3 shows the EPR spectra of calumetite recorded at different temperatures in the 123–295 K range. The spectra are spread over a region of approximately 1000 G. It is observed that as the temperatures decreases, the intensity of the resonance signal gradually increases in 123–295 K range. At room temperature, the resonance magnetic fields corresponding to g|| component is Hr = 2944 G and for g⊥ component is Hr = 3133 G. At low temperatures in 123–273 K range, these resonance magnetic fields are found to be at 2911 G (for g||) and at 3100 G (for g⊥). This shows a slight shift in the position of both the resonance signals by about 33 G toward lower field region at low temperatures as compared to those at room temperature. This slight variation may be due to a structural rearrangement

(1)

where β is the Bohr magneton, z is the symmetry axis, S and I are the electron and spin operators; Bx, By, and Bz are the magnetic field components; g|| and g⊥ are the parallel and perpendicular components of the g tensor; and A|| and A⊥ are the parallel and perpendicular components of the hyperfine tensor A. The nuclear quadrupole and nuclear Zeeman interaction terms are ignored. The solution of the spin-Hamiltonian gives the expression for the peak position of the principal “g” and “A” tensors as (Bleaney et al. 1955) hυ = gβB + mA + (15 / 4 − m2 )

A⊥2

2 gβB

hυ = g⊥βB + mA⊥ + (15 / 4 − m2 )

A2 + A⊥2 4 g⊥βB

(2a)

(2b)

for parallel and perpendicular peaks, respectively, where “m” is the nuclear magnetic quantum number of the Cu nucleus with values 3/2, 1/2, –1/2, and –3/2 and “υ” is the microwave frequency at resonance. Using Equations 2a and 2b, the spin-Hamiltonian parameters were evaluated and the g tensors are found to be g|| = 2.26 and g⊥ = 2.10. The calculated g|| and g⊥ values are characteristic of Cu2+ ions coordinated by six ligands that form an octahedron elongated along the z-axis (Ardelean et al. 2000). As

Figure 3. EPR spectra of polycrystalline calumetite at different temperatures in 123–295 K range.


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of the site distortion in calumetite as the temperature is lowered. Di Benedetto et al. (2002) also observed a variation of g values at low temperature (130 K) for Cu2+ ions in tetrahedrite-group minerals. The linewidths (∆H) of the resonance signals of Cu2+ ions for both g|| and g⊥ components are found to be 95 G and 179 G, respectively. The linewidths for both these components are found to be independent of temperature in the 123–295 K range. This indicates a strong Cu2+ spin-spin interaction in the sample. Similar results on linewidths were reported in the literature for Mn complexes (Seehra and Castner 1970; Gupta and Seehra 1970; Gupta et al. 1972). The lineshape retains the asymmetric character for all the spectra. The population difference (N) between the Zeeman levels of the calumetite sample have been evaluated at different temperatures by making use of the area under the absorption curve for Cu2+ ions with the help of the reference EPR spectra of CuSO4⋅5H2O by using the formula given by Weil et al. (1994). N=

Ax (Scan x )2 Gstd ( Bm )std ( gstd )2 [ S ( S + 1)]std ( Pstd )1/ 2 Astd (Scan std )2 Gx ( Bm ) x ( g x )2 [ S ( S + 1)]x ( Px )1/ 2

[std ]

(3)

where A is the area under the absorption curve, which was obtained by double integrating the first derivative absorption curve, Scan is the magnetic field corresponding to unit length of the chart, G is the gain, Bm is the modulation field width, g is the g factor, S is the spin of the system in the ground state, and P is the power of the microwave. The subscripts x and std represent the corresponding quantities for the calumetite sample and the reference sample, respectively. The g value for Cu2+ ions is taken as (g|| + 2g⊥)/3 from the EPR data. The population difference (N) at room temperature for Cu2+ is found to be approximately of the order of 1020. The population difference gradually increases with decreasing temperature, the phenomenon that is expected for transition metal ions in minerals. A graph between log N and reciprocal of absolute temperature (1/T) is shown in Figure 4. Normally one would expect a linear relationship between log N and 1/T for transition metal ions present in a mineral (Gopal et al. 2004) due to the Boltzmann distribution law. Such a phenomenon is not observed in the present work. This non-linear relationship may be due to the CuCl2 species that are prevailing at high-Cu concentrations, together with minor traces of paratacamite in the sample (Leofanti et al. 2000). It was also mentioned by Leofanti et al. (2000) that the signal reflecting spherical symmetry around Cu2+ nucleus can be attributed to CuCl2 clusters. In summary, we present here the first spectroscopic study on a natural sample of calumetite by means of EPR and DTA methods. The DTA trace shows two endothermic reactions at about 378 and 598 K, which are attributed to dehydration of the sample. Data from TG analyses indicate the weight loss of the sample up to 24 wt% in the temperature region of 600 to 700 K. The EPR spectrum at room temperature exhibits resonance signals characteristic of Cu2+ ions in distorted octahedral sites. The intensity of the resonance signal is found to increase gradually with decreasing temperature. Temperature-dependent EPR studies reveal a slight shift of the position of the resonance signals toward lower field region, which is attributed to the structural rearrangement of the site distortion. The population difference (N) between the Zeeman levels for the Cu2+ ions is found to

Figure 4. A plot of log N vs. 1/T for the Cu2+ ions in calumetite.

increase with decreasing temperature. The plot of log N vs. 1/T shows a non-linear variation, which is attributed to the CuCl2 ions that are prevailing at high-Cu concentrations, together with minor traces of paratacamite in the sample. The linewidths (∆H) for both g|| and g⊥ components are found to be independent of temperature indicating strong Cu2+ spin-spin interactions.

Acknowledgments The authors are grateful to Richard Wilkin and anonymous reviewers for their critical comments, as well as constructive suggestions, which helped us to improve the manuscript. G.N.H.K. is thankful to Y.C. Ratnakaram, Department of Physics, Sri Venkateswara University, Tirupati, for his constant support and encouragement.

References cited Ardelean, I., Peteanu, M., Ciceo-Lucacel, R., and Bratu, I. (2000) Structural investigation of CuO containing strontium-borate glasses by means of EPR and IR spectrometry. Journal of Materials Science: Materials in Electronics, 11, 11–16. Blanco, J., Fayos, J., Garcia De la Banda, J.F., and Soria, J. (1973) Study of supported copper chloride catalysts by electron paramagnetic resonance and X-ray diffraction. Journal of Catalysis, 31, 257–263. Bleaney, B., Bowers, K.D., and Ingram, D.J.E. (1955) Paramagnetic resonance in diluted copper salts. I. Hyperfine structure in diluted copper tutton Salts. Proceedings of the Royal Society A, 228, 147–157. Chao, C.C. and Lunsford, J.H. (1972) EPR study of copper(II) ion pairs in Y-type zeolites. Journal of Chemical Physics, 57, 2890–2897. Christopher, K. and Kurt, P. (2003) Lime blue: A mediaeval pigment for wall paintings? Studies in Conservation, 48, 171–182. Di Benedetto, F., Bernardini, G.P., Borrini, D., and Emiliani, C. (2002) Crystal chemistry of tetrahedrite solid-solution: EPR and magnetic investigations. The Canadian Mineralogist, 40, 837–847. Gopal, N.O., Narasimhulu, K.V., and Lakshmana Rao, J. (2004) Optical absorption, EPR, infrared and Raman spectral studies of clinochlore mineral. Journal of Physics and Chemistry of Solids, 65, 1887–1893. Gupta, R.P. and Seehra, M.S. (1970) Critical behavior of the paramagnetic linewidth in RbMnF3. Physics Letters A, 33, 347–348. Gupta, R.P., Seehra, M.S., and Vehse, W.E. (1972) Shift of Néel temperature and

658


EPR linewidth of KMnF3 with Mg doping, Physical Review B, 5, 92–95. Kumar, G.N.H., Parthasarathy, G., Chakradhar, R.P.S., Omkaram, I., Lakshmana Rao, J., and Ratnakaram, Y.C. (2009) Electron paramagnetic resonance studies on clinochlore from Longitudinal Valley area, northeastern Taiwan. Physics and Chemistry of Minerals, 36, 447–453. Kumar, G.N.H., Parthasarathy, G., Chakradhar, R.P.S., Lakshmana Rao, J., and Ratnakaram, Y.C. (2010) Temperature dependence on the electron paramagnetic resonance spectra of natural jasper from Taroko Gorge (Taiwan). Physics and Chemistry of Minerals, 37, 201–208. Leofanti, G., Padovan, M., Garilli, M., Carmello, D., Zecchina, A., Spoto, G., Bordiga, S., Turnes Palomino, G., and Lamberti, C. (2000) Alumina-supported copper chloride. Journal of Catalysis, 189, 91–104. Pan, Y., Nilges, M.J., and Mashkovtsev, R.I. (2008) Radiation-induced defects in quartz. II. Single-crystal W-band EPR study of a natural citrine quartz. Physics and Chemistry of Minerals, 35, 387–397. Parthasarathy, G. (2006) Effect of high-pressures on the electrical resistivity of natural zeolites from Deccan Trap, Maharashtra, India. Journal of Applied Geophysics, 58, 321–329. Parthasarathy, G., Kunwar, A.C., and Srinivasan, R. (2001) Occurrence of moganite rich chalcedony from the Deccan flood basalts, Killari, Maharashtra, India. European Journal of Mineralogy, 13, 127–134. Parthasarathy, G., Chetty, T.R.K., and Haggerty, S.E. (2002) Thermal stability and spectroscopic studies of zemkorite: A carbonate from the Venkatampalle kimberlite of southern India. American Mineralogist, 87, 1384–1389. Parthasarathy, G., Choudary, B.M., Sreedhar, B., Kunwar, A.C., and Srinivasan, R. (2003) Ferrous saponite from the Deccan Trap, India, and its application in adsorption and reduction of hexavalent chromium. American Mineralogist, 88, 1983–1988. Parthasarathy, G., Choudary, B.M., Sreedhar, B., and Kunwar, A.C. (2007) En-

vironmental Mineralogy: Spectroscopic studies on ferrous saponite and the reduction of hexavalent chromium. Natural Hazards, 40, 647–655. Punnoose, A., Seehra, M.S., Dunn, B.C., and Eyring, E.M. (2002) Characterization of CuCl2/PdCl2/activated carbon catalysts for the synthesis of diethyl carbonate. Energy and Fuels, 16, 182–188. Scott, D.A. (2000) A review of copper chlorides and related salts in bronze corrosion and as painting pigments. Studies in Conservation, 45, 39–53. Seehra, M.S. and Castner, T.G., Jr. (1970) Critical broadening of the EPR linewidth in MnF2. Solid State Communications, 8, 787–790. Sinha-Roy, S. (2000) Precambrian metallotects and mineralization types in Rajasthan: Their relation to crustal evolution. In M. Deb, Ed., Crustal Evolution and Metallogeny in the Northwestern Indian Shield, p. 217–239. Narosa Publishing Home, New Delhi. Weil, J.A., Bolton, J.R., and Wertz, J.E. (1994) Electron Paramagnetic Resonance: Elementary Theory and Practical Applications, 498 p. Wiley, New York. Williams, S.A. (1963) Anthonyite and calumetite, two new minerals from the Michigan copper district. American Mineralogist, 48, 614–619. Xiong, H., Mo, W.L., Hu, J.L., Bai, R.X., and Li, G.X. (2009) CuCl/phen/NMI in homogeneous carbonylation for synthesis of diethyl carbonate: Highly active catalyst and corrosion inhibitor. Industrial and Engineering Chemistry Research, 48, 10845–10849.

Manuscript received July 22, 2010 Manuscript accepted November 27, 2010 Manuscript handled by Richard Wilkin